Switching type power converter circuit and method for use therein

ABSTRACT

A switching type power converter circuit includes a step-down converter circuit, a DC/AC converter circuit coupled to the step-down converter circuit, and a rectifier circuit coupled to the DC/AC converter circuit. In one embodiment, the DC/AC converter operates with near 50% duty cycle and with substantially zero-voltage, substantially minimum current switching in a resonant mode. An auxiliary step down converter may be added. An AC/DC converter front end with a full-wave bridge, an RF filter, and a power factor correction circuit may also be added.

PRIORITY CLAIM

This application claims priority under 35 U.S.C. §119(e) of U.S.Provisional Application No. 60/288,047, filed on May 2, 2001, entitled“50% DUTY CYCLE RATIO, RESONANT, ZERO CURRENT, ZERO VOLTAGE, TWO STAGESWITCHING POWER CONVERTER WITH LOSS FREE SYNCHRONOUS RECTIFIER GATEDRIVE”.

FIELD OF THE INVENTION

The present invention relates to electronic power conversion circuits,and more specifically, to switching type power converter circuits.

BACKGROUND OF THE INVENTION

Many systems employ power converter circuits. These circuits receiveelectrical power in one form and convert it to another form, forexample, to a form that is usable by electrical equipment employedwithin the particular system.

One type of power converter circuit is referred to as a switching typepower converter circuit or simply a switching power supply. Switchingtype power converter circuits make use of switches, as well ascapacitors, inductors and/or transformers, in order to convert theelectrical power from one form to another. These switches have an onstate and an off state. The on state is sometimes referred to as theclosed state or the conducting state. The off state is sometimesreferred to as the open state or the non-conducting state.

As with many power converter circuits, a switching type power convertercircuit is often expected to operate with a particular level ofefficiency and to provide a particular level of regulation over line andload changes.

The efficiency of a switching type converter circuit depends in part onthe amount of power that is dissipated across the switches. The powerloss across the switches is equal to the product of the voltage acrossthe switch and the current through the switch. In this regard, thelosses during the transitions from the on state to the off state, andvice versa, are often the main design concern. (When the switch is inthe on state, the voltage across the switch is ideally zero. When theswitch is in the off state, the current through the switch is zero.)Losses can occur during the transition from the on state to the offstate, and vice versa, if there is a non-zero voltage across the switchand non-zero current through the switch. Such losses are proportional tothe product of the power lost per transition and the switchingfrequency. Therefore, to reduce the losses across a switch, azero-current condition is desired while the switch transitions from theon state to the off state, and a zero-voltage condition is desired whilethe switch transitions from the off state to the on state.

Several techniques have been introduced, which accomplish zero-voltageswitching inherently at constant switching frequency. One of thesetechniques requires a full-bridge switching arrangement with fourprimary switches in which the regulation is accomplished by shift phasemodulation. This technique has several drawbacks including the limitedavailability of phase-modulated integrated control circuits and thelarge number of parts, which include four primary switches, at least twosecondary switches and at least two large magnetic circuit elements. Thetechnique suffers from an inability to accomplish zero-voltage switchingat light loads without additional circuit elements and additionalcomplexity.

Another circuit to address this purpose is based on the single-endedforward converter that accomplishes zero-voltage switching by additionof an extra primary side switch and capacitor. Disadvantages of thisconverter include in additional voltage stress on the primary switchingelements required to reset the transformer core. The parts required aretwo large magnetic circuit elements, the transformer and the filterinductor, two primary switches, a large primary capacitor, and twosecondary switching elements.

There is one example of prior art that accomplishes a zero-voltageswitching converter, which has a single magnetic circuit element,accomplishing both magnetic energy storage and isolation. This converterrelies on high AC magnetizing fields in order to accomplish zero-voltageswitching, requiring that the magnetizing field and the magnetizingcurrent change sign during each cycle. However, these increased lossesimpose a limit on the level of power density and efficiency that can beobtained with this approach.

Notwithstanding the performance level of current switching type powerconverter circuits, further improvements are sought.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a power converterapparatus is provided. The power converter apparatus comprises astep-down converter circuit of switching type having an input port tocouple to a supply voltage and having an output port to provide anoutput voltage at a magnitude that is lower than a magnitude of thesupply voltage, and having a control circuit to receive a feedbacksignal and regulate the magnitude of the output voltage in responsethereto; a DC/AC converter circuit of switching type having a primaryside and a secondary side, the primary side having an input port coupledto the output port of the step-down converter circuit, the secondaryside having an output port to provide an AC output voltage; a rectifiercircuit having an input port and an output port, the input port beingcoupled to the secondary side of the DC/AC converter circuit, the outputport supplying a DC voltage; and a feedback circuit to generate thefeedback signal in response to the output port of the rectifier circuit.

According to another aspect of the present invention, a power converterapparatus is provided. The power converter apparatus comprises step downconverter means for receiving a supply voltage and generating an outputvoltage at a magnitude that is lower than a magnitude of the supplyvoltage, the step down converter means including means for regulatingthe output voltage in response to a feedback signal; a DC/AC convertercircuit of switching type having a primary side and a secondary side,the primary side having an input port coupled to the output port of thestep-down converter circuit, the secondary side having an output port toprovide an AC output voltage; a rectifier circuit having an input portand an output port, the input port being coupled to the secondary sideof the DC/AC converter circuit, the output port supplying a DC voltage;and a feedback circuit to generate the feedback signal in response tothe output port of the rectifier circuit.

According to another aspect of the present invention, a power converterapparatus is provided. The power converter apparatus comprises a stepdown converter means for receiving a supply voltage and generating anoutput voltage at a magnitude that is lower than a magnitude of thesupply voltage, the step down converter means including means forregulating the output voltage in response to a feedback signal; a DC/ACconverter means for receiving the output voltage of the step downconverter means and providing an AC output voltage; a rectifier meansfor coupling to the secondary side of the DC/AC converter means andsupplying a DC voltage; and a feedback means for receiving the DC outputvoltage of the rectifier means and generating the feedback signalsupplied to the step down converter means.

According to another aspect of the present invention, a method for apower converter is provided. The method comprises: receiving a supplyvoltage and generating a first output voltage having a magnitude that islower than a magnitude of the supply voltage, where the act ofgenerating comprises regulating the first output voltage in response toa feedback signal; generating an AC voltage from the first outputvoltage; rectifying the AC voltage to provide a DC voltage; andgenerating the feedback signal in response to the DC voltage.

Notwithstanding the potential advantages of one or more embodiments ofone or more aspects of the present invention, it should be understoodthat there is no requirement that any embodiment of any aspect of thepresent invention address the shortcomings of the prior art.

This invention and/or embodiments thereof will be more fully appreciatedand understood from the accompanying detailed description in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an AC/DC power supply that incorporates aDC/DC power converter according to one embodiment of the presentinvention;

FIG. 2 is a diagram of one embodiment of the AC/DC and PFC stages of theAC/DC power converter circuit of FIG. 1;

FIG. 3 is a schematic diagram of one embodiment of the step-downconverter circuit and the DC/AC circuit of the DC/DC power convertercircuit of FIG. 1;

FIG. 4 shows a representation of an equivalent circuit (from an ACviewpoint) for the step-down converter circuit and the DC/AC convertercircuit of FIG. 3;

FIG. 5 shows signal timing waveforms for one embodiment of the DC/DCpower converter circuit of FIG. 1;

FIG. 6 is a schematic diagram of one embodiment of the third stage ofthe DC/DC power converter circuit of FIG. 1;

FIG. 7 is a schematic diagram of another embodiment of the first andsecond stages of the DC/DC power converter circuit of FIG. 1; and

FIG. 8 is a schematic diagram of another embodiment of the third stageof the DC/DC power converter circuit of FIG. 1.

DETAILED DESCRIPTION

FIG. 1 shows an AC/DC power supply 100 that incorporates a DC/DC powerconverter circuit in accordance with one embodiment of the presentinvention. The DC/DC power converter circuit of the present inventionmay be used by itself, as a DC/DC power supply, or may be combined withone or more other circuits in forming other types of power supplycircuits, for example as shown in FIG. 1 to provide an AC/DC powersupply.

The AC/DC power supply 100 has three stages: an AC/DC converter stage101, a power factor control (PFC) stage 102, and a DC/DC converter stage103. The DC/DC converter stage 103 includes a step-down convertercircuit 110, a DC/AC converter circuit 111, a rectifier circuit 112 anda feedback circuit 113. The step down converter circuit 110 and theDC/AC converter circuit 111 are each switching type power convertercircuits. As stated above, switching type power converter circuits makeuse of switches, as well as capacitors, inductors and/or transformers,to convert electrical power from one form to another.

In operation, AC power, provided from an AC supply (e.g., AC supplymains, not shown), is supplied via signal line(s) (represented by asignal line 120) to the AC/DC converter stage 101. The AC/DC converterstage 101 outputs a rectified voltage, which is supplied through signalline(s) (represented by a signal line 121) to the PFC stage 102. The PFCstage 102 applies power factor correction to raise the power factor ofthe AC/DC power supply circuit 100. If the power factor correction isideal, then the power factor will reach unity and the AC/DC power supplywill appear purely resistive to the AC supply mains. However ideal powerfactor correction may not be needed or obtained in all applications. Theoutput of the PFC stage 102 is a regulated DC voltage, which is suppliedthrough signal line(s) (represented by a signal line 122) to thestep-down converter circuit 110 of the DC/DC converter stage 103. Thestep-down converter circuit 110 outputs a regulated DC voltage, themagnitude of which is lower than the magnitude of the voltage into thestep-down converter circuit. The regulated DC voltage from the step-downconverter circuit 110 is supplied through signal line(s) (represented bya signal line 123) to the DC/AC converter circuit 111. The DC/ACconverter circuit 111 converts the regulated DC voltage to an ACvoltage, which is supplied through signal lines 124 to the rectifiercircuit 112. The rectifier circuit 112 generates a DC voltage, V_(out1),which is the output voltage of the AC/DC power supply 100. The feedbackcircuit 113 receives the output voltage (through signal line(s)represented by a signal line 125) and supplies a feedback signal(through signal line(s) represented by a signal line 126) to thestep-down converter circuit 110. The feedback signal is preferably anisolated feedback signal, although this is not a requirement. Thestep-down converter circuit 110 regulates its own output voltage suchthat the output voltage V_(out1), has the desired magnitude.

As shown in FIG. 1, the output of the DC/DC power converter circuit 103also may be supplied to another step-down converter circuit 130 togenerate one or more auxiliary outputs, such as, for example,V_(out1S1), although this is not required. Auxiliary outputs may also beprovided by supplying the output of the DC/AC converter 111 to anadditional rectifier circuit 135 that is in series with a step-downconverter circuit 136.

The AC/DC power supply may be provided with one or more additional DC/DCpower converter circuits, for example as indicated at 132, to generateadditional DC output voltages, for example, such as V_(out2). Theseadditional DC/DC power converter circuits may be similar to the DC/DCpower converter circuit 103 described above.

FIG. 2 shows a further detail of one possible embodiment of the AC/DCand PFC stages 101, 102. The input to the AC/DC stage 101 is a voltage,V_(ac), which is supplied through signal lines 120A, 120B. The output ofthe AC/DC stage 101 is V_(in), which is supplied through signal lines121A, 121B to the input port of the PFC stage 102. The output of the PFCstage 102 is an output voltage V_(pfc), which is supplied through signallines 122A, 122B.

In this embodiment, the AC/DC stage 101 includes an optional filtercircuit 201 and a rectifier circuit 202. The optional filter circuit 201comprises a conventional RF filter circuit. The rectifier circuit 202has a bridge topology (not shown) to convert a sinusoidal signal to thefull wave rectified signal, V_(in).

The PFC stage 102 includes an inductor 203, a diode 204, a capacitor205, a switch 206, a sense resistor 207 and a PFC control circuit 208.The capacitor 205 is connected across the output of the PFC stage 102and supplies a current, I_(load), to a load, which in some embodimentsis constant, but is not limited to such. The inductor 203 and the diode204 are connected in series with the capacitor 205. The switch 206 and asense resistor R_(sense) are connected in series between a terminal ofthe inductor 203 and the signal line 122B. As used herein, the term“terminal” includes leads and/or nodes.

The duty cycle of the switch 206 is controlled by the control circuit208. In the on state, the switch 206 provides a shunt path (through thesense resistor R_(sense)) that causes an increase in the current in theinductor 203. In the off state, the current in the inductor relaxes(decreases). The control circuit 208 varies the duty cycle of the switch206 based on the input voltage V_(in) and average output current (whichis determined indirectly based on variations in the output voltageV_(pfc)) to obtain a DC level at the output voltage V_(pfc). If the loadis constant, then the average current through the inductor 203 is equalto the load current I_(load). As stated above, if the power factorcorrection is ideal, then the power factor will reach unity and theAC/DC power supply will appear purely resistive to the AC supply mains.

FIG. 3 shows a schematic diagram of one embodiment of the step-downconverter circuit 110 and the DC/AC converter circuit 111 of the DC/DCpower converter circuit 103 (FIG. 1). In this embodiment, the step downconverter circuit 110 includes an inductor 311, a diode 312, a switch313, a control circuit 315 and a capacitor C_(pp). The switch may, forexample, comprise a transistor (e.g., an n-channel MOSFET) and anintegral diode, represented by a diode D_(p2). The input to thestep-down converter circuit 110 is V_(pfc) (i.e., the output of the PFCstage 102 (FIGS. 1, 2)). The output of the step-down converter circuit110 is V_(hb), which in this embodiment, appears across a capacitorC_(pp).

A first terminal of the inductor 311 is coupled to a first terminal ofthe capacitor C_(pp), a second terminal of which is coupled to thesignal line 122A. The second terminal of the inductor 311 is coupled toa first terminal of the switch 313, a second terminal of which iscoupled to the signal line 122B. The diode 312 couples the secondterminal of the inductor to the signal line 122A. The control circuit315 receives a feedback signal from the feedback circuit 113 (FIG. 1)and supplies a signal to control the state of the switch 313. Thecontrol circuit 315 may, for example, comprise an MC33364 manufacturedby Motorola.

As used herein, the term “coupled to” includes connected directly to andconnected indirectly to (i.e., through one or more elements).

In operation, the control circuit 315 varies the on/off frequency of theswitch 313 based on the feedback signal, to thereby vary the voltageacross the capacitor C_(pp) so as to obtain the desired output voltage,V_(OUT1), at the output of the DC/DC power converter circuit 103.

As stated previously, losses in a switch can be reduced by establishinga zero-voltage condition across a switch while the switch transitionsfrom the off state to the on state. Consequently, some embodiments mayinclude features to establish a zero (or near zero) voltage conditionacross the switch 313 at turn on of the switch. The phrase “turn on ofthe switch” refers to the instant at which the switch starts totransition from the off state to the on state. For example, in thisembodiment, the step-down converter circuit 110 operates in adiscontinuous mode in which the magnetic flux through the inductor 311is periodically reset to 0. This is referred to as critical conductionmode. More particularly, the current through the inductor 311 riseswhile the switch 313 is in the on state, and resets to zero while theswitch is in the off state. In addition, the voltage across capacitorC_(pp), i.e., V_(hb), is chosen to be at least ½ of V_(pfc). Forexample, in this embodiment, V_(hb) is chosen to be equal to about ¾ ofV_(pfc). The use of critical mode conduction in combination with the useof V_(hb) greater than or equal to about ½ of V_(pfc), results in azero-voltage condition across the switch 313 at turn on of the switch313. This helps minimize the losses in the switch 313.

Some embodiments may not provide zero-voltage switching but maynonetheless provide substantially zero volts switching. Substantiallyzero volts means that the voltage across the switch is less than orequal to 10% of the maximum voltage observed across the switch while theswitch is in the off state. Note that if the integral diode across theswitch is forward biased then the voltage across the switch will beclamped and will be substantially zero volts.

It should be understood that other embodiments need not use criticalconduction mode. The step-down converter can work with fix frequency orvariable frequency. If the fix frequency mode is chosen, then afrequency equal to the DC/AC converter or a multiple of this frequencyis a good choice, in order to have less interference and audible noisebetween these to stages. The disadvantage of a fix frequency mode isthat it cannot achieve zero voltage across the switch without specialtechniques.

The variable frequency mode which is also called the critical conductionmode is relatively straightforward to implement and may have a higherefficiency than the fixed frequency mode. One disadvantage however, isthat the variable frequency mode may result in a large frequencyvariation (e.g. 200 KHz . . . 700 KHz) as the load changes from maximumto minimum. A large frequency variation can make it more difficult tocompensate the loop to achieve stability across all line and loadconditions.

The voltage V_(c), which is a fraction of V_(pfc), establishes theamount of energy available to supply to the load in the event that theAC power to the AC/DC power supply 100 is removed. Thus, in someembodiments, V_(c), is selected so as to be able to provide a “hold uptime” that is long enough for the system to perform any requiredmaintenance prior to power down.

The DC/AC converter circuit 111 includes a bridge circuit 300, atransformer 318, and a capacitor assembly that includes capacitorsC_(ps1), C_(ps2). The bridge circuit 300 includes a pair of switchesS_(p1), S_(p2). Each of the switches may, for example, comprise atransistor (e.g., an n-channel MOSFET). In this embodiment, switchS_(p1), has an integral diode, indicated at D_(p1) and a parasiticcapacitance, represented at C_(sp1). Similarly, switch S_(p2) has anintegral diode, indicated at D_(p2) and a parasitic capacitance,represented at C_(sp2).

The bridge circuit 300 is coupled across the output of the step-downconverter stage, which provides a voltage V_(hb). More particularly, afirst terminal of the bridge circuit 300 is coupled to the firstterminal of the capacitor C_(pp). A second terminal of the bridgecircuit 300 is coupled to the second terminal of the capacitor C_(pp).

The capacitors C_(ps1), C_(ps2) are connected in series with oneanother, the series combination being connected in parallel with thecapacitor C_(pp).

The transformer 318 includes a primary side winding, indicated at L_(p),and two secondary side windings, indicated at L_(s1A) and L_(s2A). Aleakage inductance L₁ and a resistance R_(sfs) are also shown.

A first terminal of the primary side winding of the transformer 318 iscoupled to an output of the bridge circuit 300. A terminal of theprimary side winding is coupled to node “b” between the capacitorsC_(ps1) and C_(ps2). The secondary windings provide the AC outputvoltage from the DC/AC converter circuit 111.

The DC/AC converter circuit further includes a control circuit 321 whichsupplies a respective control signal to each of the switches S_(p1),S_(p2). In this embodiment, the control signals each have a fixedfrequency and a near 50% duty cycle. The term “duty cycle” refers to theon time (i.e., the amount of time that a switch is commanded to the onstate) divided by the period of a switching cycle, i.e.,T_(on)/(T_(on)+T_(off)). The phrase “near 50%” means greater than orequal to 40%. Note that the switches S_(p1), S_(p2) usually exhibit someamount of turn off delay, and therefore it may be desirable to keep theduty cycle less than or equal to 48%, so as to reduce the possibility ofcross conduction (where switches S_(p1), S_(p2) are both on at the sametime). The control circuit may be implemented in any manner. Someembodiments may use an MC34067 manufactured by Motorola, although thisis not required for the present invention.

The duty cycle may be fixed, although this is not required. However,operating the DC/AC converter circuit at a fixed frequency and a fixed,near 50% duty cycle ratio helps reduce output ripple across thecapacitor C_(out). This in turn helps reduce the size of the smoothingfilter, discussed below, and thereby helps improve efficiency and loopstability.

For purpose of the operational state analysis, it is assumed that thefilter capacitor 309 is sufficiently large so that the voltage developedacross the capacitor is approximately constant over the switchinginterval. The transformer 318 windings may have a coupling coefficientclose to unity. The resulting leakage inductance is called L₁. Theinternal resistance of the primary winding is usually close to zero soit will be ignored. Furthermore it will be recognized that, while only asingle output version is considered in this analysis, multiple outputvoltages may be obtained through the addition of windings, switches,synchronous rectifier gate drives, rectifier diodes and capacitorsoperated as herein to be described.

The operation of the circuits in FIG. 3 are further described below withrespect to FIG. 4.

FIG. 6 shows one embodiment of the rectifier circuit 112 of the DC/DCpower converter circuit 103 (FIG. 1). The rectifier circuit 112 includesa bridge circuit 600 and a capacitor C_(out). The bridge circuit 600includes two synchronous rectifier switching gate drive devices S_(s1),S_(s2). A first terminal of the switch S_(s1) is coupled to a firstterminal of the secondary L_(s1A), a second terminal of which is coupledto a first terminal of the secondary L_(s2A). A first terminal of theswitch S_(s2) is coupled to a second terminal of the secondary L_(s2A).A first terminal of the capacitor C_(out) is coupled to the center tapof the secondaries L_(s1A), L_(s2A). A second terminal of the switchS_(s1) is coupled to a second terminal of the capacitor C_(out) and tothe second terminal of the switch S_(s2).

In operation, the near 50% duty cycle ratio synchronuos gate drivecircuit 412, operates the devices from the secondary side: S_(s1),S_(s2) substantially simultaneously with the devices on the primaryside: S_(p1), S_(p2).

If synchronous gate drive switches are used in the rectifier's placewithin the third stage, a bidirectional power flow from primary side tosecondary side and vice-versa is obtained due to the synchronization ofthe switching pairs (S_(p1), S_(s1)) and (S_(p2), S_(s2)). In thisrespect, if V_(pfc) is zero and an external voltage is applied to theoutput (V_(out)), a proportional voltage (V_(hb)) will be found in theprimary side.

FIG. 6 also shows the secondary windings L_(s1A), L_(s2A) from the DC/ACconverter circuit 111 (FIG. 3), a feedback circuit 608, and an optionalsmoothing circuit 610. The feedback circuit 608 receives the output andsupplies a feedback signal to the step-down converter stage 110 (FIG.3). The optional smoothing circuit 610 includes a choke (inductor) 606and a capacitor 607 that are connected in series between the center tapof the secondaries L_(s1A), L_(s2A) and the output of the bridge circuit600. The capacitor 607 is coupled across the output, V_(out).

It has been determined that the magnitude of the output filter choke 606may be relatively small compared to that regularly used in power supplyoutput stages. This is in part due to the near 50% duty cycle ratioemployed in this embodiment of the DC/AC converter circuit. A smalloutput filter choke has two potential advantages. First, it helpsminimize the current path for high current outputs, thereby enhancingthe overall efficiency. Second, it implies a small phase shift for theoutput filter, and therefore control loop stability and response isgreatly enhanced.

In this embodiment, the impedance of the capacitor is relatively smallcompared to the impedance of the output filter choke 606. For example,Z_(C) may be less than or equal to Z_(L)/20 at a frequency equal to 2times the operating frequency of the DC/AC converter circuit. The outputfilter operates at two times the frequency of the DC/AC convertercircuit because for each switching cycle of the DC/AC converter circuit,there are two switching cycles in the rectifier circuit (one cycle foreach of S_(s1), and S_(s2)).

FIG. 4 shows a representation of an equivalent circuit (from a dynamicor AC viewpoint) for the step-down converter circuit 110 and the DC/ACconverter circuit 111 shown in FIG. 3. The equivalent circuit can beanalyzed from this dynamic or AC point of view in terms of two states.

In a first state, one of the switches S_(p1), S_(p2) is in the on state.As to the primary side, there is a series circuit formed by: L₁ (theequivalent leakage inductance of the transformer 318), L_(p) (theprimary winding of the transformer 318) and an equivalent capacitorC_(ps) having a value equal to the sum of C_(ps1) and C_(ps2). In thisembodiment, the value of C_(pp) is considerably larger than the value ofC_(ps) and the variation of the voltage across C_(out) (at nominalcurrent of the output) is relatively small comparing to the outputvoltage, during an interval of time equal to the sum of T_(on) andT_(off) (see FIG. 5). In addition, it has been assumed that themagnitude of the ripple voltage in point “b” 402 is small enough,compared to the magnitude of V_(hb), to be ignored in this analysis,although the present invention is not limited to such.

As to the secondary side of the transformer 318, one of the rectifyingswitches S_(s1) or S_(s2) is also closed (see FIG. 6). As stated above,if the value of capacitor C_(out) is large enough at the workingfrequency (of the DC/AC converter circuit), then one of the secondarywindings L_(s1A) or L_(s2A) will be short circuited by the low impedanceof capacitor C_(out) from an AC point of view. Consequently, the primarywinding L_(p) will up present an impedance that is almost zero. A seriescircuit is thus formed by the leakage inductance L₁ and C_(ps).

As stated previously, losses in a switch can be reduced by establishinga zero-current condition while the switch transitions from the on stateto the off state. As such, some embodiments may include features toestablish a zero-current condition at turn off times of the switch. Thephrase “turn off times of the switch” refers to the instant at which theswitch starts to transition from the on state to the off state.

Some embodiments may not provide zero current switching but maynonetheless provide substantially minimum current switching.Substantially minimum current means that the current through the switch,at the instant that the switch is turned on, is less than or equal to20% of the maximum current observed through the switch under nominaloutput power conditions.

In order to achieve a substantially minimum-current turn off for theswitches, the relationship between the working frequency (F) of theDC/AC converter circuit and the value of the two mentioned componentsshould be:

F=1/(2*π*{square root over (L1*Cps))}

The resulting timing for the current waveform in the primary winding oftransformer 318 is shown in FIG. 5.

With the component values selected as described above, the frequency ofthe drive circuit 321 may be set equal to or approximately equal to theresonant frequency. Thereafter, in the operation of the circuit, thedrive circuit 321 turns the switches off and on at the resonantfrequency and the desired resonant behavior is automatically provided bythe circuit. As a result, at the moment that a switch on the primaryside is turned off, the current through that particular switch is at itsminimum value. Note that in this embodiment, this current is not equalto zero. Because the current on the secondary side is related to thecurrent on the primary side, the current on the secondary side is alsoat its minimum.

In a second state, both S_(p1) and S_(p2) are open. In order to reducelosses when one of the above switches is going to be turned ON, azero-voltage condition on that switch is desired at the turn on time.This condition can be fulfilled if:

(1) There is enough magnetizing energy stored in the primary transformerwinding L_(p) during a conduction period of time (marked as Ton in FIG.5) in order to modify the potential of point “a” (401) from thepotential at node “c” (403) to the potential at node “d” (404) orvice-versa, and

(2) There is enough dead time (marked as Toff in FIG. 5) between themoment when one of the switches S_(p1) or S_(p2) goes off and the otherone goes on, to get a zero-voltage condition at the turn on time.

These two conditions can be reformulated in the following way:

T=2*(Ton+Toff)

Csp=Csp1+Csp2

Cps=Cps1+Cps2

The magnetizing value of the current through L_(p) at the end of theconduction period (Ton) is:

Ipmag=Vhb/Lp* Ton

During T_(off), a resonant circuit is formed by L_(p) and the parasiticcapacitors connected to node “a” (401), referred to as C_(sp). Themagnetizing energy of L_(p) is transferred to C_(sp) and from theequivalence of the energies we have the following formula:

Lp*Ipmag²/2≧Csp*Vhb²/2

Lp≦Ton² /Csp

Toff≧½*2*π*{square root over (Lp*Csp)}

Lp≦Toff²/(π² *Csp)

Based on the above relations, we can estimate the values for the L_(p),L₁, T_(on) and T_(off).

Zero voltage switching is achieved as a result of transferring themagnetic energy to the parasitic capacitance at node “a” as follows.First, due to the symmetrical switching waveform, it can be assumed thatthe DC voltage across C_(ps1) and C_(ps2) are is equal to another. Thus,the potential of node “b” is approximately ½ of V_(hb) (as measuredagainst node “c”). Furthermore, assuming that S_(p2) is on, the voltageon node “a” is zero relative to node “c”. The current in S_(p2) ramps upand the magnetic energy in the core increases accordingly. If S_(p2) isturned off, then the magnetizing energy stored in the primary of thetransformer is delivered to node “a” until the potential of point “d” isreached. With the potential at node “a” equal to the potential of node“d”, the voltage across S_(p1) is zero.

To get the desired amount of energy delivered to node “a”, the value ofthe inductance is chosen in relation to the total equivalent capacitanceat node “a” measured against node “c”. In practice, the value of theinductance is typically chosen to be no greater than the critical valueso as to make sure that the potential of node “a” moves fully from thepotential at node “c” to the potential at node “d”, or vice versa. Ifthe energy is at the critical value then the integral diodes will not beforward biased. If the energy is greater than the critical value, thenthe integral diodes will become forward biased and clamp the voltage.The off time (dead time) should be larger than the amount of time neededto transfer the magnetizing energy from the transformer into theparasitic capacitors connected to node “a” (for example the parasiticcapacitors of the switches). However, the greater the energy, the lesstime is needed to move the potential at node “a” from the potential atnode “c” to the potential of node “d” or vice versa. Thus, the off timemay be reduced.

In order to improve manufacturability, it may be desirable to addadditional capacitance to node “a” so that any variations in thecapacitance of the parasitic capacitors will be small compared to theoverall value of capacitance connected to node “a”. Note that anotherpractical issue with respect to parasitic capacitors of the switches(especially MOSFET devices) is that they are often dependent on theapplied voltage. Adding additional capacitance to node “a” allowsaccurate control of the voltage change observed on node “a”.

The above analysis has deliberately ignored the energy that is stored inthe leakage inductance. This energy is proportional with the powertransferred through the transformer 318 and is relevant for the analysisof the second state (i.e., where both S_(p1), and S_(p2) are open),where this energy is to be added to the magnetizing energy of thetransformer. If the resonance condition of the series circuit mentionedabove is fulfilled, then this energy is small compared to the chosenmagnetizing energy, at the end of a conduction cycle (the current I_(p)is almost zero). If the circuit is out of resonance, then thisadditional energy will speed up the transitions of node “a”, between thepotentials of nodes “c” and “d”. In this case, a circuit that willdynamically modify the T_(off) time accordingly to the load can be usedto enhance the overall timing and finally the efficiency.

Another possible approach is to remove the capacitor C_(pp) andsynchronize the near 50% duty cycle drive circuit that is working atfrequency F with the step-down converter circuit that can work at thefrequency F or multiples of F in order to get a noiseless conversion.This approach may improve the overall response of the feedback loop,although operating the step-down converter at a fixed frequency may makeit more difficult to achieve a zero-voltage condition across theswitching element 313.

Some embodiments employ a single equivalent capacitor in place of thecapacitors C_(ps1), C_(ps2). The single equivalent capacitor may beconnected between the primary of the transformer and either of signallines 123A, 123B (FIG. 3). However, the use of both of the capacitorsC_(ps1), C_(ps2) helps optimize the response of the loop with respect tolarge transitions in the load current.

FIG. 5 shows signal timing waveforms for one embodiment operating with anear 50% duty cycle ratio and in a substantially minimum current (notethat “zero-current” may be ideal but may not achieved in this particularembodiment) resonant mode. The waveforms include waveforms for Ip(representing the current through the primary without the magnetizingcomponent), V_(a) (the voltage at node “a” 401 measured with respect tothe signal line 122B), V_(ss1), and V_(ss2) (the voltage across theswitches in the rectifier circuit), V_(gate) S_(p1), S_(s1) (the controlsignal applied to the switches S_(p1) and S_(p2)) and V_(gate) S_(p2),S_(s2) (the control signal applied to the switches S_(s1), S_(s2)).

Some embodiments may provide soft starting, under/over-voltageprotection and/or current limiting. Such features can be provided in anyof various ways. For example, for soft starting, the duty cycle for theswitch 313 may be limited to a low value at power up and then allowed toincrease to its steady state value. Alternatively, if the feedbackcircuit 608 uses an internal reference, that reference may be limited toa low value at power up and then allowed to increase to its steady statevalue. Over-voltage may be handled as follows. If an over-voltagecondition is detected, the control circuit 315 may cease to turn off theswitch 313. This causes the output voltage to decrease to 0. In someembodiments, the output voltage remains at 0 until the unit is cut offfrom the AC mains and then reconnected. As to current limiting, if thefeedback circuit 608 detects that the current through the load has reachits maximum allowable value, the feedback signal to the control circuit315 may be modified as appropriate, in order to limit the averagecurrent through the inductor L_(fs) to a value that corresponds to amaximum load current desired in the secondary side. The output voltagedecreases accordingly.

FIGS. 7-8 show alternative embodiments for the step-down convertercircuit, the DC/AC converter circuit, and the rectifier circuit. Inthese embodiments, the DC/AC converter circuit and the rectifier circuiteach employ a full wave bridge configuration. Note that the DC/ACconverter circuit and the rectifier circuit shown in FIGS. 3, 6 employhalf wave configurations. A full wave configuration may provideadvantages when operating at higher output powers, where there aresmaller currents (I_(p)/2) through switches and the primarytransformer's winding, although the full wave configuration may requirea higher number of components and a more complicated control drivecircuit, as compared to the half wave configuration.

Some embodiments of one or more aspects of the present invention mayoperate in a frequency range between 10 KHz and 1 MHz.

Although disclosed above with respect to an embodiment that incorporatesvarious features that alone or in combination with one other may helpreduce cost and improve efficiency and/or reliability, it should beunderstood that the present invention is not limited to such. Forexample, there is no requirement to employ power factor correction orzero-voltage and zero-current switching. In addition, there is norequirement to operate the DC/AC converter circuit and the rectifiercircuit at a fixed frequency and fixed duty cycle near 50%. Nor is thereany requirement to use a small output filter choke. Furthermore, whilethe embodiments disclosed above do not employ snubber circuits, there isno prohibition against such circuits. Moreover, although some featuresand techniques are described as optional, this is not meant to implythat all other features and techniques are required, i.e., not optional.

Note that, except where otherwise stated, terms such as, for example,“comprises”, “has”, “includes” and all forms thereof, are consideredopen-ended so as not to precluded additional elements and/or features.

Also note, except where otherwise stated, phrase such as, for example,“in response to”, “based on” and “in accordance with” mean “in responseat least to”, “based at least on” and “in accordance with at least”,respectively, so as, for example, not to preclude being responsive to,based on, or in accordance with more than one thing.

As used herein, a “port” has one or more leads or nodes, but is nototherwise limited to any particular structure.

While there have been shown and described various embodiments, it willbe understood by those skilled in the art that the present invention isnot limited to such embodiments, which have been presented by way ofexample only, and that various changes and modifications may be madewithout departing from the spirit and scope of the invention.Accordingly, the invention is limited only by the appended claims andequivalents thereto.

What is claimed is:
 1. Power converter apparatus comprising: a step-downconverter circuit of switching type having an input port to couple to asupply voltage and having an output port to provide an output voltage ata magnitude that is lower than a magnitude of the supply voltage, andhaving a control circuit to receive a feedback signal and regulate themagnitude of the output voltage in response thereto; a DC/AC convertercircuit of switching type having a primary side and a secondary side,the primary side having an input port coupled to the output port of thestep-down converter circuit, the secondary side having an output port toprovide an AC output voltage; a rectifier circuit having an input portand an output port, the input port being coupled to the secondary sideof the DC/AC converter circuit, the output port supplying a rectifiedvoltage; and a feedback circuit to generate the feedback signal inresponse to the output port of the rectifier circuit.
 2. The powerconverter apparatus of claim 1 wherein the DC/AC converter circuitoperates at a fixed frequency.
 3. The power converter apparatus of claim2 wherein the DC/AC converter circuit operates at a duty cycle that isnear 50%.
 4. The power converter apparatus of claim 2 wherein the DC/ACconverter circuit operates in a resonant mode.
 5. The power converterapparatus of claim 2 wherein the duty cycle of the DC/AC convertercircuit is fixed.
 6. The power converter apparatus of claim 1, furthercomprising a drive circuit that supplies first control signals to theDC/AC circuit and supplies second control signals to the rectifiercircuit, said first control signals being synchronous with said secondcontrol signals.
 7. The power converter apparatus of claim 1 wherein theDC/AC converter circuit has a bridge circuit and a transformer, thebridge circuit having an input port and an output port, the input portof the bridge circuit being coupled to the input port of the DC/ACconverter circuit, the output port of the bridge circuit being coupledto a primary side winding of the transformer, the transformer furtherhaving a secondary side winding coupled to the output port of the DC/ACconverter circuit.
 8. The power converter apparatus of claim 7 whereinthe bridge circuit has a switch connected between the input port of thebridge circuit and the output port of the bridge circuit, the switchhaving an on state and an off state.
 9. The power converter apparatus ofclaim 8 further comprising a drive circuit that supplies a controlsignal to turn the switch of the DC/AC converter circuit on and off, andmeans for establishing substantially zero volts across the switch atturn on times of the switch.
 10. The power converter apparatus of claim9 wherein the input port of the DC/AC converter has a first node and asecond node and the means for establishing substantially zero voltsacross the switch at turn on times of the switch comprises a firstcapacitance and a second capacitance connected in series between thefirst node and the second node of the input port to the DC/AC converter.11. The power converter apparatus of claim 9 wherein the means forestablishing substantially zero volts across the switch at turn on timesof the switch comprises a capacitance that is coupled to a lead of theprimary side winding of the transformer.
 12. The power converterapparatus of claim 8 further comprising a drive circuit that supplies afirst control signal to turn the switch of the DC/AC converter circuiton and off, and supplies a second control signal to turn the switch ofthe rectifier circuit on and off, and means for establishingsubstantially minimum current through the switch at turn off times ofthe switch.
 13. The power converter apparatus of claim 8 wherein therectifier circuit has a bridge circuit and a filter, the bridge circuitof the rectifier circuit having (i) an input port coupled to the outputport of the DC/AC converter circuit and (ii) an output port coupled toan input port of the filter, the bridge circuit further having a switchthat is connected between the input port of the bridge circuit and theoutput port of the bridge circuit, the power converter apparatus furthercomprising a drive circuit that supplies a first control signal to turnthe switch of the DC/AC converter circuit on and off, and supplies asecond control signal to turn the switch of the rectifier circuit on andoff synchronously with turn on and turn off of the switch of the DC/ACconverter circuit.
 14. The power converter apparatus of claim 1 whereinthe rectifier circuit has a bridge circuit and a filter, the bridgecircuit of the rectifier circuit having (i) an input port coupled to theoutput port of the DC/AC converter circuit and (ii) an output portcoupled to an input port of the filter, the bridge circuit furtherhaving a switch connected between the input port of the bridge circuitand the output port of the bridge circuit.
 15. The power converterapparatus of claim 14 further comprising a drive circuit that supplies acontrol signal to the switch of the rectifier circuit and means forestablishing substantially zero volts across the switch at turn on timesof the switch.
 16. The power converter apparatus of claim 15 wherein themeans for establishing substantially zero volts across the switch atturn on times of the switch comprises a capacitance.
 17. The powerconverter apparatus of claim 16 wherein the capacitance has a firstterminal coupled to the input port to the filter and has a secondterminal coupled to the output port of the bridge circuit.
 18. The powerconverter apparatus of claim 14 further comprising means forestablishing substantially minimum current through the switch at turnoff times of the switch.
 19. The power converter apparatus of claim 14wherein the filter comprises an inductor and a capacitor each having animpedance, wherein the rectifier circuit has a switching frequency andwherein the impedance of the capacitor at the switching frequency issmall relative to the impedance of the inductor at twice the switchingfrequency of the DC/AC converter circuit.
 20. The power converterapparatus of claim 1 wherein the control circuit of the step downconverter circuit is coupled to a switch that is coupled to an inductor,the control circuit having an output port to supply a control signal tocontrol the duty cycle of the switch to regulate the magnitude of theoutput voltage of the step down converter circuit.
 21. The powerconverter apparatus of claim 20 wherein said inductor is coupled betweenthe switch and the output port of the step down converter circuit, andwherein the current in said inductor increases while the switch is in anon state and decreases while the switch is in an off state.
 22. Thepower converter apparatus of claim 21 wherein the current in saidinductor decreases to zero while the switch is in an off state.
 23. Thepower converter apparatus of claim 7 wherein the bridge circuit is afull wave bridge.
 24. The power converter apparatus of claim 7 whereinthe bridge circuit is a half wave bridge.
 25. The power converterapparatus of claim 13 wherein the bridge circuit is a full wave bridge.26. The power converter apparatus of claim 13 wherein the bridge circuitis a half wave bridge.
 27. The power converter apparatus of claim 1further comprising: a second rectifier circuit having an input port tocouple to an AC supply voltage and having an output port to provide arectified voltage, the output port of the second rectifier circuit beingcoupled to the input port of the step-down converter circuit.
 28. Thepower converter apparatus of claim 27 further comprising: a power factorcorrection circuit having an input port coupled to the output port ofthe second rectifier circuit and having an output port coupled to theinput port of the step-down converter circuit.
 29. Power converterapparatus comprising: step-down converter means for receiving a supplyvoltage and generating an output voltage at a magnitude that is lowerthan a magnitude of the supply voltage, the step down converter meansincluding means for regulating the output voltage in response to afeedback signal; a DC/AC converter circuit of switching type having aprimary side and a secondary side, the primary side having an input portcoupled to the output port of the step-down converter circuit, thesecondary side having an output port to provide an AC output voltage; arectifier circuit having an input port and an output port, the inputport being coupled to the secondary side of the DC/AC converter circuit,the output port supplying a rectified voltage; and a feedback circuit togenerate the feedback signal in response to a signal provided at theoutput port of the rectifier circuit.
 30. Power converter apparatuscomprising: step-down converter means for receiving a supply voltage andgenerating an output voltage at a magnitude that is lower than amagnitude of the supply voltage, the step down converter means includingmeans for regulating the output voltage in response to a feedbacksignal; DC/AC converter means for receiving the output voltage of thestep down converter means and providing an AC output voltage; rectifiermeans for coupling to a secondary side of the DC/AC converter means andsupplying a rectified voltage; and feedback means for receiving therectified voltage of the rectifier means and generating the feedbacksignal supplied to the step down converter means.
 31. A methodcomprising: receiving a supply voltage and generating a first outputvoltage having a magnitude that is lower than a magnitude of the supplyvoltage, where the act of generating comprises regulating the firstoutput voltage in response to a feedback signal; generating an ACvoltage from the first output voltage; rectifying the AC voltage toprovide a rectified voltage; and generating the feedback signal inresponse to the rectified voltage.
 32. The method of claim 31 whereingenerating the AC voltage comprises driving switches in a bridge circuitto an on state and to an off state at a fixed frequency.
 33. The methodof claim 32 wherein generating the AC voltage comprises driving switchesin a bridge circuit to an on state and to an off state at a duty cyclenear 50%.
 34. The method of claim 32 wherein generating the AC voltagecomprises driving switches in a bridge circuit to an on state and to anoff state at a fixed duty cycle.
 35. The method of claim 31, whereingenerating the AC voltage comprises driving switches in a first bridgecircuit to an on state and to an off state and wherein rectifying the ACvoltage comprises driving switches in a second bridge circuit to an onstate and to an off state synchronous with the driving of the switchesin the first bridge circuit.
 36. The method of claim 31 whereingenerating the AC voltage comprises driving a switch on and off andestablishing substantially zero volts across the switch at turn on timesof the switch.
 37. The method of claim 31 wherein rectifying the ACvoltage comprises driving a switch on and off and establishingsubstantially minimum current through the switch at turn off times ofthe switch.
 38. The method of claim 31 wherein rectifying the AC voltagecomprises driving a switch on and off and establishing substantiallyzero volts across the switch at turn on times of the switch.
 39. Themethod of claim 31 wherein generating the AC voltage comprises driving aswitch on and off and establishing substantially zero volts across theswitch at turn on times of the switch.
 40. The power converter apparatusof claim 1 wherein the rectified voltage of the rectifier circuit is aDC voltage.
 41. The power converter apparatus of claim 8 wherein therectified voltage of the rectifier circuit is a DC voltage.
 42. Thepower converter apparatus of claim 14 wherein the rectified voltage ofthe rectifier circuit is a DC voltage.
 43. The power converter apparatusof claim 21 wherein the rectified voltage of the rectifier circuit is aDC voltage.
 44. The power converter apparatus of claim 29 wherein therectified voltage is a DC voltage.
 45. The power converter apparatus ofclaim 30 wherein the rectified voltage of the rectifier circuit is a DCvoltage.
 46. The method of claim 31 wherein the rectified voltage is aDC voltage.